At age 43, Sheryl Sandberg has built an impressive resume that includes high-ranking positions at Google and the U.S. Department of the Treasury. Yet Sandberg -- author of the new book Lean In: Women, Work and the Will to Lead -- admits that when she began talking with Facebook CEO Mark Zuckerberg about becoming the chief operating officer for the world's largest social network, she had some fears about accepting the job.

In this clip from "Oprah’s Next Chapter," Sandberg confesses to Oprah her fears about working at Facebook, saying that they kept her up at night. "Would Mark and I get along? Would we have the relationship we’d need to have?" Sandberg remembers wondering. "It's a very unique relationship where he runs the company and I run part of the company, working for him. We need to trust each other. You know, he's 15 years younger than I was."

The 15-year age difference between Sandberg and Zuckerberg was apparent before she was even hired at Facebook, Sandberg says in the clip. "When I was interviewing with him, one night we were supposed to talk -- it was 9 o'clock at night," she says. "He called and he said, 'I'll [have to] call you later.' I said, "Well, I’m going to go to sleep in about 30 minutes so if you're still at your dinner, we'll talk tomorrow. He calls the next day and is like, 'Are you okay? Were you sick?' I said, 'No, I wasn't sick.' He said, 'You went to bed at 9:30.'"

Sandberg laughs now, saying that going to bed at 9:30 is just what she does, but admits that at the time, the conversation sparked another fear. "I definitely had this fear that I was going to be too old," she says. "It was scary at first."